Friday, January 20, 2017

A rapid and specific assay for P. Acnes, will it help in patient management?

These authors have developed a polymerase chain reaction (PCR)- restriction fragment length polymorphism (RFLP) approach that sensitively and specifically identifies P acnes in tissue specimens within a 24-hour period.

They designed primers to amplify a unique region of the 16S rRNA gene in P acnes that contained a unique HaeIII restriction enzyme site. PCR and RFLP analyses were optimized to detect P acnes DNA in in vitro cultures and in arthroscopic surgical biopsy specimens from patients with P acnes infections.

They derived a 564 base-pair PCR amplicon from all of the known P acnes strains. HaeIII digests of the amplicon yielded a restriction fragment pattern that was unique to P acnes. P acnes-specific amplicons were detected in as few as 10 bacterial cells and in clinical biopsy specimens of infected shoulder tissues.

They concluded that this PCR-RFLP assay combines the sensitivity of PCR with the specificity of RFLP mapping to identify P acnes in surgical isolates. The assay is robust and rapid, and a P acnes-positive tissue specimen can be confirmed within 24 hours of sampling, facilitating treatment decision making, targeted antibiotic therapy, and monitoring to minimize implant failure and revision surgery.

The authors remind us that P acnes DNA can theoretically be derived from living or dead P acnes cells. Thus, this technology has the weakness of being unable to distinguish between live or dead P acnes cells, whereas the current approach of anaerobic cultures will only identify live P acnes .

Furthermore, the PCR-RFLP analyses described in this report is qualitative and not quantitative. Thus, if P acnes is a low-level commensal of the tissue being assessed (eg, the deep dermis), this assay will not distinguish between a P acnes infection and the presence of P acnes as a nonpathogenic commensal.

The strength of this extremely sensitive technology is the ability to detect fewer than 10 P acnes cells in the sample. The weakness is that this level of sensitivity makes aseptic sample collection very important.

Comment: This an exciting step forward. As the authors stated, the risk in using a very sensitive technique is that it does not reflect whether the bugs are alive or dead or the load of bacteria in the shoulder (see this link). While the 24 hours required for P. Acnes identification is much faster than the 17 days recommended for culture results, it is still not fast enough to inform intraoperative decisions, such as the need for prosthesis exchange. Finally, this assay is specific for P. Acnes, while it is recognized that other strains of Propionibacterium may be pathogenetic (see this link).

It will be of great interest to learn of the clinical utility of this test.